Euro Bike

RAY: An American serviceman stationed in Europe decides to buy a motorcycle. At the shop, the owner, seeing the uniform, says, "If you're shipping this back to the States at the end of your tour, there's something you'll have to change before you can drive it there."

"Is it something I can buy from you?" asks the serviceman.

"No, I don't have one," says the owner, "but you'll certainly have no trouble getting it when you get home."

What did the serviceman have to change to make his bike legal to drive in America?